vair68robert,
Starting with a pair of 2.2s in 1996, I too have mostly bought used since then, and it's still been a rather expensive (but rewarding) hobby. I'm surprised and pleased to learn that CA-2300s are fetching $5k+ used. That's a bit more than I paid used for my CT-2300 in 2011.
I admire your DIY effort upgrading your electronics. I lack expertise and skill with a soldering iron in that area, but I did take a deep-dive into acoustic measurement and room treatment as well as designing and building an ultrasonic vinyl cleaning system to earn my DIY wings.
Back on the topic of adding subs, I should add that Thiel cross-overs, whether passive or active, do a superb job seamlessly integrating subs with main speakers.
Bob